Excretion and Resorption
The kidneys receive approximately 1220 ml of blood each minute, from which 125 ml/min of glomerular filtrate is formed in the average male. Thus, 180 L of glomerular filtrate is formed each day, of which only 1.5 to 2.0 L is excreted as urine. Therefore, every day at least 178 L is resorbed by the kidneys, and only about 1% of he total glomerular filtrate is excreted [2].
The fluid component from the blood passes through the filtration barrier to become the ultrafiltrate. The endothelial cells and basement membrane of the glomerulus as well as the podocytes of Bowman's capsule make up the filtration barrier. Blood enters the glomerular capillaries from an afferent arteriole and leaves through an efferent arteriole. Vasoconstriction of this efferent arteriole creates a high hydrostatic pressure in the glomerular capillary, forcing water, ions, and small molecules through the filtration barrier into Bowman's capsule. Whether a substance is filtered depends on both its molecular size and charge [5].
Most resorption of materials from the ultrafiltrate occurs in the proximal tubule. Normally, the following amounts are absorbed in the proximal tubule: 100% of proteins, amino acids, and creatine; almost 100% of bicarbonate ions; 67% to 80% of sodium and chloride ions; and 67% to 80% of the water [2]. Tables 3 and 4 below summarize the major function of all components of the uriniferous tubule.
The excretory passages of the kidney consist of the minor and major calyces and the pelvis of the kidney which pass waste products along to the ureter, the single urinary bladder and subsequently on to the single urethra.Endocrine Function
The kidney plays a role in the endocrine system by producing four hormones as described below [5].
1. Renin is produced by the juxtaglomerular apparatus which causes the formation of angiotensin II. Angiotensin II acts on the proximal tubules to promote sodium retention and also acts as a potent vasoconstrictor.
2. Vitamin D is a steroid hormone metabolized by the kidney to the active form 1,25-dihydroxycholecalciferol, which promotes calcium and phosphate absorption from the gut.
3. Erythropoietin is a protein produced in the kidney which promotes red blood cell formation in bone marrow.
4. Prostaglandins are also produced in the kidney and have various effects, especially on renal vessel tone.
Because the kidneys are poised to sense plasma concentrations of compounds such as sodium, potassium, hydrogen ion, oxygen, and glucose, they are also important regulators of homeostasis including regulation of blood pressure, glucose metabolism, and erythropoeisis (the process by which red blood cells are produced).
